Adtran is a well-known vendor in the telecom community, but as people start moving small business from traditional phones and Key or Hybrid Key/PBX systems into the exciting and considerably less reliable (during the first 30 days or so) world of VoIP, with truly converged DIA (direct internet access) and VoIP, Adtran is getting better known, because they are a great vendor.
1) 10 years support/warranty out of the box with no additional contract.
ZOMGWTF! Ask Cisco how much that costs for smartnet for an IAD for 10 years. You could buy a couple of them as spares for that.
2) 95% of the functionality, 50% of the price.
We all know Cisco gear is pricey, but is it really better? Technically, yes. SNMP support is more robust on the Cisco device, if you want to be able to do a GET NEXT on a table that lists your current firewall sessions, Cisco's MIB's will support that, Adtran's won't. They didn't spend quite as much time/money engineering the ability to pull every minute detail out it via SNMP.
Another mild failure is that in some very outlying ancient protocol support environments, the Adtran IAD gear just won't cut it. It's very rare though, again, 95%.
M0stly, though, the important stuff is there. Again, this is a price/performance opinion, not a line by line comparison of who's better or not.
About the same or better. Much more streamlined support organization. Instead of having to wait 30-60 minutes waiting for a callback, and attempt to give all kinds of technical details to someone who clearly hears everything you say as "blahblahblahblahblah", you get into a support queue fast, and if you have an issue, the guys aren't bad. If it goes over the tech's head, or has hit a bug, it goes right into engineering. There's about as much visibility into engineering with Adtran as there is into Cisco, but it's a lot faster to get access to.
Who uses them? All kinds of people. Probably the most notable one I can think of is Verizon. They use Adtran extensively throughout the network. If Adtran made a competitive product to the Actiontec routers verizon pushes into everyone's house with FIOS, Adtran would probably have that business. They also make some more hardcore telecom gear, because that's where they come from, the telecom business. CSU/DSU's, MUX's, all kinds of ubiquitous gear you find at a phone company.